Sadly yes, you cannot do that. Nor can you erase all contents and settings. The only thing I can recommend is to uninstall your tweaks from Cydia (very carefully), reboot and see if that helps. Maybe you have conflicting modifications.

No you cannot do that. You can remove the tweaks that you installed. If you want to remove their cache too, download iCleaner tweak and remove the cache and cydia junk files and once you have done that, remove the iCleaner too.